Method of protecting ferrous strip in hot-dip processes

ABSTRACT

A METHOD FOR COATING BATCH ANNEALED FERROUS STRIP WITH MOLTEN METAL BY A HOT-DIP PROCESS IN WHICH FLUXING IS ELIMINATED. THE STRIP IS INITIALLY COATED WITH A VERY THIN METALLIC LAYER BY ELECTROPLATING OR VAPOR DEPOSITION AND THEN, AFTER SUITABLE ACTIVATION OF THE SURFACE, PASSED THROUGH A MOLTEN METAL COATING BATH.

May 1, 1973 J. N. LAIDMAN METHOD OF PROTECTING FERROUS STRIP 1N HOT-DIPPROCESSES 2 Sheets-Sheet l Filed Oct. 29, 1970 INVENTOR L/O/I)? /V.lo/oman 0w ATTORNEY ECO May 1, 1973 J. N. LAIDMAN METHOD OF PROTECTINGFERROUS STRIP [N HOT-DIP PROCESSES 2 Sheets-Shet 2 Filed Oct. 29, 1970INVENTOR Jo/m 1V Zak/man United States Patent Ofiice 3,730,758 METHOD OFPROTECTING FERROUS STRIP IN HOT-DIP PROCESSES John N. Laidman,Coopersburg, Pa., assignor to Bethlehem Steel Corporation Filed Oct. 29,1970, Ser. No. 85,011 Int. Cl. C23c 1 US. Cl. 117-51 5 Claims ABSTRACTOF THE DISCLOSURE A method for coating batch annealed ferrous strip withmolten metal by a hot-dip process in which fluxing is eliminated. Thestrip is initially coated with a very thin metallic layer byelectroplating or vapor deposition and then, after suitable activationof the surface, passed through a molten metal coating bath.

BACKGROUND OF THE INVENTION This invention relates to the coating ofmetal stock, and more particularly to hot-dip coating of steel strip.

Hot-dipping is the term applied to processes in which a metal base, suchas cold-rolled steel strip, is coated by passing a clean strip through abath of a molten metal. When the coating metal is zinc, the process isknown as galvanizing. The effectiveness of a metal such as zinc as aprotective coating for steel strip depends to a considerable extent uponthe uniformity and adherence of the coating material. To achieveuniformity and adherence of a metal coat, the ferrous base must be cleanand maintained free of oxide formation immediately prior to the coatingstep.

Cold-rolled strip is customarily annealed prior to coating in order torender it more ductile and suitable for various forming operations, andthe like, subsequent to coating. One widely used method of cleaning andmaintaining steel strip free from oxides prior to hot-dip coating is bycontinuously, in-line annealing the strip before it enters the coatingbath. During annealing the strip is raised to, or above, 1200". F.Subjection to this high temperature is effective to burn off allcontaminants, and, if accomplished in a reducing atmosphere will alsoeffectively remove all oxides from the surface of the strip. The stripmay then be passed from the protective reducing, or neutral, atmospheredirectly into the hot-dip coating bath without intermediate contact withoxygen in the atmosphere.

Steel strip which has to withstand various deep drawing or other severeforming operations, such as that encountered in the forming of fendersfor automobiles, must be exposed to the heat of the annealing operationfor periods ranging from up to several hours or even days. It isimpractical to perform this type of annealing in an in-line operation.Such strip is, therefore, batch annealed. While the strip could, afterbatch annealing, be directed Through a conventional in-line annealingand coating line to further clean and then coat the strip, such aprocedure is impractical because the high temperature necessary toassure cleaning impairs the mechanical properties obtained by batchannealing. Batch annealed strip, therefore, is customarily first cleanedwith various acids in pickling or electrolytic cleaning baths and thentreated with halide salts known as fluxes. Various other reactivesubstances in addition to halide salts may also be used as fluxes, buthalides are the most popular. The =flux layer serves to protect thestrip surface from oxidation by the atmosphere as the strip passes fromthe cleaning tanks to Patented May 1., 1973 the molten bath, andparticularly at the point where the strip is preheated several hundreddegrees prior to entrance into the coating bath. Preheating of the stripis customary in continuous operations to attain a satisfactory adherenceof the coating and prevent excessive cooling of the coating bath.

Attempts have been to eliminate the use of flux. These attempts haveincluded the use of protective atmospheres and the like through whichthe strip passes between the cleaning tanks and the hot-dip bath. One ofthese prior attempts has included the use of hydrochloric acid fumes totry to protect the strip as it passes from the pickling or cleaning tankto the molten galvanizing bath. This expedient has, however, not provenpractical.

Fluxing, while providing a satisfactory manner of achieving desiredcleanliness and strip protection, has the disadvantage of generatingcorrosive fumes. In recent years, due to increased production oncontinuous galvanizing lines, increases in fume formation have causedrising incidents of equipment corrosion, environmental hazards, and airpollution. Fluxing, furthermore, has had the disadvantage of failing toprovide a uniformly tight, adherent coat. Typical defects caused by fluxfailure include black spots, smear, and uncoated areas.

None of the prior art methods afford a method of protection for ferrousstrip in hot-dip processes which has. the advantages of fluxing, e.g.adaptability to batch annealed strip, but none of the disadvantages,such as corrosion and air pollution.

SUMMARY OF THE INVENTION 1 have discovered that the aforementionedproblems of the prior art may be obviated by the two step coatingprocess of this invention. Step one comprises a flash coating of metaldeposited on a clean steel strip by vacuum deposition, electroplating,or similar processes. This first coating step is followed by aconventional hot-dipping operation in which a second coat of the same,or different, metal is applied to the metal strip. By flash coating, Imean a very thin layer of metal coating. In the example of galvanizing,a first flash zinc coating is applied to a clean steel strip. The coatedstrip is then preferably activated either by the application to thestrip of a dilute inorganic acid wash, or other suitable agents, afterwhich the strip is passed through a molten zinc bath. The temperature ofthe molten zinc bath is such that the first flash zinc coat is meltedand either (a) forms an alloy with the ferrous strip or (b) iscompletely removed and replaced by the second hot-dipped zinc coat.

BRIEF DESCRIPTION OF THE DRAWING FIGS. 1 and 1a diagrammatically depictthe preferred embodiment of my invention.

FIG. 1b is an alternate embodiment.

FIG. 10 shows a further variation of my method.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIG. 1, 36 inchwide, 26 gage batch annealed, cold-rolled rimmed steel strip enters acleaning bath 10, which contains a suitable alkali cleaner of the typewell known in the art, followed by scrubbing unit 12 and rinse tank 14.The strip continues through pickling bath 16, which contains an acidsuch as 10-14% aqueous hydrochloric acid. After pickling, the strip isscrubbed in scrubbing unit 18 and rinsed in tank 20. After leaving tank20, and referring now to FIG. la, the strip enters electroplating tank22 Where the strip is plated with zinc from an ionic dispersion of zincin a zinc cyanide bath to a depth of .02 oz. per sq. ft. (.01 oz. ofzinc/sq. ft. on each side). Emerging from electroplating tank 22 thestrip proceeds from there to heating unit 26 in which the temperature ofthe strip is raised from about 400 F. to about 600 F. Leaving heatingunit 26 the strip passes through activating unit 28 where the surface issprayed with 1% aqueous hydrochloric acid to prepare the strip surfacefor subsequent coating, and from there enters galvanizing bath 30. Thepot temperature of galvanizing bath 30 is about 850 F. and the bathcontains, in addition to molten zinc, about 0.13% aluminum. Aftergalvanizing the strip passes through gas wipers 32 to establish thedesired thickness of coating. An alternate of my preferred method wouldbe to have a second activating unit (not shown) immediately afterelectroplating tank 22 and before heating unit 26.

Another embodiment of my method is shown in FIGS. 1 and 1b. The stripcleaning process as shown in FIG. 1 is performed, as described above,but instead of the electroplating as shown in the FIG. 1a, a vapordeposition process is employed. The strip leaves rinse tank 20, shown inFIG. 1, enters drying unit 33 followed by vapor deposition unit 34, asshown in FIG. 1b where the strip is coated with zinc from a moleculardispersion of vaporized zinc. Leaving vapor deposition tank 34, thestrip enters heating unit 36, and from there passes through tank 38which contains a suitable non-fuming, activating fluid such a moltensodium or potassium halide or the like. The activating fluid floats uponthe surface of molten zinc 39 in the area under tank 38. Thank 38 ispositioned directly over galvanizing pot 40 and the strip passesdirectly into galvanizing pot 40 from tank 38. After emerging fromgalvanizing pot 40, the coated strip passes through wipers 42. The flashmetal coating protects the strip in place of the conventional fluxcoating and the molten halide bath activates the strip surface and meltsthe first coat just prior to immersion of the strip in the metal coatingbath. The molten halide activating fluid operates without the generationof any noxious or corrosive fumes.

Another alternative method by which my invention may be practiced isshown in FIGS. 1, c and 1c. The strip cleaning process as shown in FIG.1, and previously explained, is performed, after which the strip enterselectroplating unit 22 in which a flash metal coat is applied, as inFIG. 1a and previously described. The strip emerges from electroplatingunit 22 and enters heating unit 44 as shown in FIG. 10. In heating unit44 the temperature of the strip is raised from about 400 F. to about 600F. While the strip is simultaneously subjected to a reducing atmosphere.Such an atmosphere may be formed by adjusting the gases in the heatingunit to provide products of combustion such as carbon monoxide, carbondioxide, hydrogen and water vapor. Alternatively chlorine or hydrogenchloride may be used to activate the surface of the strip preparatory tofurther coating. The strip preferably remains in the protective reducingatmosphere until passing into, and through, galvanizing pot 46. Theemerging strip then passes through wipers 48.

Since the initial flash coating applied to the strip is designed only toprovide short terms or temporary protection to the strip surface, itdoes not need to be as thick as would be necessary for more permanentprotection. The flash coating can thus be applied relatively cheaply ina simple coating apparatus having only a feW electroplating cells orvapor deposition units.

Many variations are possible within the scope of my invention. Since thePrimary purpose of the first thin coat is strip protection, any processwhich applies a thin metallic protective coat is applicable. An exampleof this would be sherardizing, a process in which particulate metalparticles of dust-like fineness are used as the coating medium. Passagethrough a fluidized bed of a particulate metallic coating material mightalso be taken advantage of to apply the first flash protective layer.

Further, since the heating and activating process steps performedbetween the coating steps are all directed to preventing strip surfaceoxidation, overcoming the effects of such oxidation, and/or preparingthe surface for the hot dip coating, any oxide reducing treatment whichaccomplishes this activation function is applicable in my method. Forexample, electroplating which is performed in a halide bath such as zincchloride would permit elimination of activation altogether as a separatestep. The halide salts remaining on the strip after the strip leaves theelectroplating unit would in this case perform the activation in situ asthe strip passes through the preheater 26 as shown in FIG. In.

My method is especially applicable to galvanizing because of themagnitude of the corrosion problem caused by fluxing fumes in continuousgalvanizing lines. It would be equally applicable, however, to hot-dipaluminum or tin coating lines. Also, while it is preferable in many, orperhap most, cases to have the initial flash protective coating consistof the same metal as the desired final hotdip metal coating, to preventthe coating bath from contamination by the protective coating as itmelts or merges with the final hot-dip coating, the two coatings may bedissimilar but compatibile metals. Examples of the latter are: tin ascoat one, with tin or zinc as coat two; or zinc as coat one with terneas coat two.

A first flash protective coating of aluminum, for in stance, may in somecases be particularly useful in gelvanizing since an aluminum additionis normally made to galvanizing baths in order to decrease interfacealloy formation of the zinc with the surface of the strip. Theprotective coating, furthermore, is so thin, about .001 inch to about.000001 inch, that only a very minor amount will enter the body of themolten coating bath.

The method of my invention has many advantages over prior art hot-dipprocesses. First and most importantly, flux, with its inherent corrosionand environmental problems, may be eliminated as a step in hotdipprocesses, especially galvanizing. My method retains, however, theadvantages afforded by a fluxing process. These advantages includeapplicability to batch annealed steel strip, economy of installation andupkeep, and the production of a superior product. Steel strip galvanizedin accordance with my method shows a bright, well-defined spanglepattern.

My process is dilferent from conventional two-coat metal coatingprocesses in which one layer of metal is applied to a strip, followed bya second layer applied to the first layer in order to form a pluralityof final coating layers on the strip surface. In the process of myinvention, the first flash metal coating acts as a temporary protectivecoating only, and is melted or alloyed and replaced by the secondhot-dipped coat. The present invention is likewise not similar tocertain reclaiming operations wherein defective tin plated sheets,otherwise unsuitable for sale, have been dipped either in a hot tin bathto make tin plate or a lead-tin mixture to make terne plate. In theselatter instances, the first coat is initially defective and serves nofunction, protective or otherwise; the only object of redipping being torecoat and thus reclaim an initially defective sheet.

I claim:

1. A method of hot-dip coating ferrous comprising:

(a) depositing a protective metal coating on a ferrous strip from aparticulate dispersion of the coating metal,

(b) heating the strip to from about 400 F. to about (c) activating theprotective surface by means of an oxide reducing treatment,

((1) passing the strip through a molten metal bath to melt theprotective metal coating and apply to the strip a single metal layercomprising metal from the molten bath.

strip 2. A method of hot-dip coating according to claim 1 wherein theoxide reducing treatment comprises subjecting the heated strip to areducing gas.

3. A method of hot-dip coating according to claim 2 in which thereducing gas is chlorine.

4. A method of hot-dip coating according to claim 2 in which thereducing gas is hydrogen chloride.

5. A method according to claim 1 wherein both the protective metalcoating and the molten metal bath are substantially comprised of zinc.10

References Cited UNITED STATES PATENTS 936,637 00/1909 Kirk 11771 M X 152,374,926 5/1945 Fink 1171 14 A X 6 1,816,617 7/1931 Schueler 204-38.51,133,628 3/1915 Field et al. 204-385 FOREIGN PATENTS 350,469 6/1931Great Britain 20438.5

OTHER REFERENCES Handbook of Chemistry & Physics, 32nd ed., 1950,Chemical Rubber Publishing C0., pp. 598-599.

General Chemistry, by 'Sisler et al., 1949, pp. 413-414.

ALFRED L. LEAVITI, Primary Examiner J. R. BATTEN, 111., AssistantExaminer US. Cl. X.R.

l17-22, 71 M, 114 A, 114 B, 114 C, 114 R; 204-38 S

